The present invention relates to a device for changing, in discrete steps, the operation timing of an intake valve or an exhaust valve according to the speed of rotation of an internal combustion engine.
Combustion chambers of four-cycle engines have intake and exhaust valves for supplying an air-fuel mixture and for discharging a combustion gas according to engine operation cycles. These valves are normally urged in a closing direction by valve springs disposed around the respective valve stems. The intake and exhaust valves can be forcibly opened against the resiliency of the valve springs by cams integrally formed on a camshaft that is driven by the crankshaft of the engine through a belt and pulley mechanism.
There have been proposed a variety of engine arrangements in which each engine cylinder is provided with a plurality of intake or exhaust valves. In such engine arrangements, when the engine operates at a low speed, one of the intake or exhaust valves is actuated, and when the engine operates at a high speed, all of the intake or exhaust valves are actuated. At the same time, the operation timing of the valves is varied according to the rotational speed of the engine for thereby increasing the efficiency of charging the air-fuel mixture into the combustion chamber over a wide range of engine operating conditions. One such device for changing the valve operation timing in an internal combustion engine is disclosed in Japanese Laid-Open Patent Publication No. 61-19911 filed by the applicant of the present application.
The device for changing the valve operating timing disclosed in this publication is operated by a power source which may be the pressure of the lubricating oil supplied by an oil pump operatively coupled to the engine crankshaft. Therefore, the valve timing changing device is subject to electrical control by a solenoid-operated valve which controls the flow of the lubricating oil.
If the solenoid-operated valve or the electric control circuit for controlling the solenoid-operated valve fails, the valve timing changing device cannot be controlled in the high engine speed range, and the valves must be operated in the low speed mode even if the engine speed is high. When this occurs, the balancing between the ignition timing and the air-fuel ratio may deviate from optimum values, thereby resulting in inefficient engine operation.
In view of the aforesaid problems attendant with such prior art arrangements, it is an object of the present invention to provide a valve operation timing changing device for an internal combustion engine, which is capable of maintaining relatively stable engine operation conditions even when an electric control circuit, or a solenoid-operated valve, malfunctions.